This invention relates to a magnetic disk drive and more particularly to a magnetic disk drive capable of securely holding magnetic disks made of a brittle non-metalic material such as glass or ceramics such that they are not broken easily either during the operation of the disk drive or because of the difference in thermal expansion between its spindle and its spacers.
Many types of magnetic disk drives have been proposed and developed. The prior art magnetic disk drive of the so-called Winchester type, as an example, has a plurality of magnetic disks stacked one on top of another with spaces 2 inserted in between on the outer peripheral surface of a spindle hub (hereinafter referred to simply as "spindle") 3 as shown in FIG. 1. The spindle 3 contains a motor 4 inside with its shaft 5 held stationary so that its exterior rotates around this stationary shaft 5. A coil is wound around this stationary shaft 5 to form a stator 6 and a magnet 7 is disposed on the internal wall of the spindle 3 so that the spindle 3 can undergo a fast rotary motion (of about 3600 rpm, for example) as the motor 4 is activated. The magnetic disks 1 are usually of the type with a magnetic film covering a metallic base plate, for example, of aluminium or an aluminium alloy and the stacked disks 1 are pressed together against a bottom rim 10 of the spindle 3 by a metallic cap 9. The rim 10 is a protrusion unistructurally formed on the external peripheral surface of the spindle 3 at its lower end. The cap 9, which is hereinafter also referred to as a fastening means, is affixed to the top surface of the spindle 3 with set screws 8 so that the disks 1 are prevented not only from racing because of their own angular momentum but also from changing their relative positions within the disk drive. The spacers 2 are made of aluminium or an aluminium alloy, the spindle 3 is made of a kind of steel, and numeral 11 in FIG. 1 indicates bearings.
Recently, there have been increasing demands for an improved recording density. In response to such demands, magnetic disks with substrates made of brittle non-metallic materials such as glass and ceramics are currently being considered because these materials have certain superior qualities. Substrates made of such materials have fewer surface defects than those of aluminium or an aluminium alloy and a magnetic film can be formed without any very hard base layer of Ni-P, altimite or the like. Mere replacement of metallic substrates by glass or ceramic substrates, however, cannot respond to the aforementioned demands because magnetic disks made of such brittle materials tend to break easily. Metallic substrates are stronger and can withstand external forces applied thereon. Brittle materials such as glass and ceramics are weaker against external forces and tend to break easily for the following two major reasons. Firstly, the surfaces of the spacers 2, the metallic cap 9, and the rim 10 may not be finished well and localized forces may develop and be applied to the disks. Secondly, the spindle 3 of the type containing a motor inside is usually made of a soft steel with coefficient of thermal expansion about 90-120.times.10.sup.-7 /deg and that of glass or ceramic base plates is about 40-120.times.10.sup.-7 /deg while that of aluminum and aluminum alloys of which the spacers 3 and the cap 9 are made is much greater, being about 230-240.times.10.sup.-7 /deg. When there is a large change in the ambient temperature, therefore, extraordinarily strong forces may be exerted on such brittle disks. These two reasons are, of course, not always clearly distinguishable. There may be occasions wherein the magnetic disks break as a combined result of these two reasons.